Why are PBE Players so ridiculously bad?

That's my face when I see their gameplay ^

I'm not going to name names, but here are some of the things I've noticed :

1. I once saw Orianna who refused to use her ultimate on multiple targets, even when they were all clumped together for several seconds. She would rather use it to last hit a retreating champion. By then she was the only left alive because they had lost the teamfight.

2. You'd be surprised by the number of PBE players who think maxing Abilities in Q, W, E order is the right way for ALL champions.

3. Some PBE junglers do not upgrade their jungle item and die multiple times while clearing. They show themselves before ganking and then run back into the brush to reattempt the gank.

4. Many PBE players do not last hit. They do try, but it is very obvious they lack experience.

I thought PBE is supposed to be the testing grounds for future content. Do you think the feedback from these kind of players is reliable?